Diesel engines, some gasoline fueled engines, and many hydrocarbon fueled power plants are operated at higher than stoichiometric air to fuel mass ratios for improved fuel economy. Such lean-burning engines and other power sources, however, produce a hot exhaust with a relatively high content of oxygen and nitrogen oxides (collectively, NOx). In the case of diesel engines, the temperature of the exhaust from a warmed up engine is typically in the range of 200 degrees to 400 degrees Celsius, and has a typical composition, by volume, of about 10% oxygen, 1.6% carbon dioxide, 0.1% carbon monoxide, 180 ppm hydrocarbons, 235 ppm NOx and the balance nitrogen and water. The NOx gases, typically comprising nitric oxide (NO) and nitrogen dioxide (NO2), are difficult to reduce to nitrogen (N2) because of the high oxygen (O2) content in the hot exhaust stream.
Reduction of NOx constituents in the exhaust of lean-operating engines remains a problem in the implementation of high fuel efficiency lean-burn engines. Two different approaches are promising: (a) NOx storage from the exhaust stream and reduction to nitrogen and (b) ammonia addition to the exhaust enabling selective catalytic reduction. The efficiency of NOx removal in both practices is significantly enhanced by an increased proportion of NO2 in the total NOx. Since engine-out NOx typically includes less than 10% NO2, NO oxidation performance of the oxidation catalysts is important for the overall treatment of NOx emissions. Currently precious metal-based catalysts are most commonly used for NOx oxidation for diesel application. But these platinum-group catalysts suffer from high material cost and insufficient durability.
Perovskites are a broad class of compounds of the general composition ABX3, where ‘A’ and ‘B’ are two complementary cations of very different sizes, and X is an anion that bonds to both. Some perovskite compositions such as LaCoO3 are known for oxidation activity with respect to volatile organic compounds. It is an object of this invention to adapt suitable perovskite materials as oxidation catalysts, such as for the treatment of lean burn engine vehicle exhausts.
Platinum-group metals, dispersed on high surface area alumina support particles, have been applied as coatings (called “washcoats”) on surfaces of cordierite or silicon carbide support bodies for catalytic treatment of automotive vehicle exhaust streams. Preferred current practice is to use a honeycomb-like, flow-through, extruded cordierite support with up to 400 parallel, open-ended gas channels per square inch and an open frontal area of about 71%. The extruded cordierite body may have a circular or oval cross-section with a suitable length for the intended reaction, and is inserted in the exhaust stream so that the hot gas flows through the channels and is affected as intended by the noble metal/alumina catalyst particle-bearing surfaces.
It is a further object of this invention to provide a practice for forming and simultaneously applying effective particulate coatings of perovskite catalyst materials (e.g., oxidation catalyst) on support bodies for exhaust gas treatment. This object includes (but is not limited to) forming in-situ and applying particles of a perovskite catalyst material on the small longitudinal wall surfaces of the cells of an exhaust gas flow-through ceramic support body.